Design for Settlement of Pile Groups by the Unified Design Method: A Case History

A series of tall apartment buildings was planned to be built on reclaimed ground over a thick deltaic deposit near-shore deposit outside City of Pusan in South Korea. The soil profile consisted of approximately 30 to 50 m of soft clay, silt, and sand on sandy gravel extending to bedrock at about 100 m depth. The deep foundation system normally used in Korea consists of steel pipe piles driven to significant toe bearing in dense soils or on bedrock. Because of the anticipated significant costs of this solution, a more economical alternative foundation system was essential, and the alternative of the PHC pile, a pretensioned spun high strength concrete pile, was proposed. To evaluate the feasibility of a PHC pile alternative, a comprehensive test programme was carried out, encompassing dynamic tests, long-term monitoring of negative skin friction, laboratory pilot tests, static loading tests on instrumented test piles, and settlement analysis. The analysis of the test data required study of strain effects from hydration and swelling of concrete, and of development of residual load before static testing, as well as of distribution of resistance along the pile due to applied load, and development of negative skin friction from settling soil and resulting drag load. Reliable estimation of pile group settlement was a key issue. Five methods for calculation of pile group settlement were compared and applied to the actual foundation layouts, of which one, the Unified Design Method, could include the effect of ongoing consolidating of the soft, compressible clay layer, interaction of adjacent foundations, and factual distribution of pile shaft resistance.